D-psicose is a monosaccharide known as a rare sugar because it is rarely found in natural materials or is present in small amounts. D-psicose has an ultralow energy density and a sweet taste similar to sugar, and thus is widely used as a functional sweetener.
D-psicose is an epimer of fructose and has a degree of sweetness and taste very similar to fructose. However, unlike fructose, D-psicose is scarcely metabolized in the body and thus has almost zero-calories. D-psicose can be used as an effective ingredient for diet foods since D-psicose has capabilities in inhibiting activity of an enzyme involved in lipid synthesis and reducing abdominal obesity. Further, sugar alcohols such as xylitol and the like that are widely used as sugar substitute may have side effects such as causing diarrhea in case of over consumption. On the contrary, D-psicose is known to have substantially no side effects (Matsue, T., Y. Baba, M. Hashiguchi, K. Takeshita, K. Izumori, and H. Suzuki. 2001. Dietary D-psicose, a C-3 epimer of D-fructose, suppresses the activity of hepatic lipogenic enzymes in rats. Asia Pac. J. Clin. Nutr. 10:233-237; Matsuo, T., and K. Izumori. 2004. D-psicose, a rare sugar that provides no energy and additionally beneficial effects for clinical nutrition. Asia Pac. J. Clin. Nutr. 13:S127).
For such a reason, D-psicose draws keen attention as a diet sweetener, and there is a growing need for development of a method for effectively producing D-psicose in the food industry. As the necessity for developing D-psicose has been raised, many researches for producing D-psicose from fructose using the conventional biological methods have been performed. As enzymes capable of converting fructose into D-psicose, D-psicose 3-epimerase derived from Agrobacterium tumefaciens and D-tagatose 3-epimerase derived from Pseudomonas cichorii or Rhodobacter sphaeroides are known. D-psicose 3-epimerase is known to have higher activity than D-tagatose 3-epimerase.
Strains belonging to genus Corynebacterium are industrial microorganisms which produce chemical materials including L-lysine, L-threonine and various nucleic acids having diverse usages in feed forage, pharmaceuticals, foods, and the like. Such strains of genus Corynebacterium are GRAS (Generally Recognized As Safe) strains, and have properties that are easy to be genetically engineered and mass cultured. Moreover, genus Corynebacterium strains have high stability under various process conditions and a relatively strong cell membrane structure as compared with other bacteria. For these reasons, the strains have biological properties that the bacterial cells exist in a stable state under high osmotic pressure owing to high sugar concentration and the like.